MAGNETIC-PROPERTIES ON NANOCRYSTALLINE COFENBB ALLOYS

The influence of alloy composition and heat treatment on the structure and magnetic properties of rapidly quenched CoFeNbB alloys were studi ed. The saturation magnetization, J(s), the magnetostriction constant, lambda(s) and the Curie temperature, T-c, of the as-quenched amorphou s alloys decrease with increasing Nb or B content (3 < X-Nb < 7, 8 < X -B < 15 at.%). Both J(s), and lambda(s), of CoFeNb7B9 show maxima at a bout 29 at.% of Co. The crystallization of all the alloys takes place in two steps. On the first step bcc-Fe(Co) grains develop in the amorp hous matrix. With increasing Nb content their grain sizes substantiall y decrease. A nanocrystalline structure with the grain size of 10-15 n m is obtained for X-Nb = 7 at.%. The saturation magnetization increase s on nanocrystallization. The magnetostriction constant, lambda(s), is positive for the as-quenched as well as nanocrystalline alloys. Its m agnitude first decreases and then remains constant or increases with f urther annealing. A slight decrease of the coercive field, H-c, at the early stage of nanocrystallization is followed by a subsequent gradua l increase. The best soft magnetic properties were observed for the na nocrystalline Co25Fe59Nb7B9 alloy. The following parameters: J(s) = 1. 58 T, lambda(s) = 25.1 ppm and H-c = 26.5 A/m, were obtained in this a lloy after annealing at 600 degrees C.